As many have pointed out on this forum, these various timescales do have
very specific meanings which often fade at levels coarser than a few
nanoseconds (modulo 1 second), and which at times are misapplied at the
1-second and higher level.

GPS Time is technically an "implicit ensemble mean".  You can say it exists
inside the Kalman Filter at the GPS Master Control Station as a linear
combination of corrected clock states.  But there is no need for the control
computer to actually compute it as a specific number, and that's why it is
implicit.  Every GPS clock is a realization of GPS Time once the receiver
applies the broadcast corrections.   GPS Time is steered to UTC(USNO), and
generally stays within a few nanoseconds of it, modulo 1 second.  UTC(USNO)
approximates UTC, and so it goes.

The most beautiful reference to GPS Time is "The Theory of the GPS Composite
Clock" by Brown, in the Proceedings of the Institute of Navigation's 1991
ION-GPS meeting.  But others, including me, routinely publish plots of it.

--Original Message-----
From: Leap Seconds Issues [mailto:[EMAIL PROTECTED] On Behalf Of
Ashley Yakeley
Sent: Tuesday, January 09, 2007 2:22 AM
Subject: Re: [LEAPSECS] Introduction of long term scheduling

On Jan 8, 2007, at 22:57, Steve Allen wrote:

> GPS is not (TAI - 19)

What is GPS time, anyway? I had assumed someone had simply defined GPS to be
TAI - 19, and made the goal of the satellites to approximate GPS time, i.e.
that GPS and TAI are the same (up to isomorphism in some "category of
measurements"). But apparently not?
Are the satellite clocks allowed to drift, or do they get corrected?

Ashley Yakeley

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